when ____ in arctic regions,they construct huts as temporary winter shelters.

题目

when ____ in arctic regions,they construct huts as temporary winter shelters.
A travel B traveled C traveling D they traveled
these compounds could not have formed if the chemical reaction _____ stopped.
A has been B was C would have been D had been
第二题到底应该是哪个答案呢？

答案

C traveling 表示状态,时间状语
D had been 虚拟语气